In a geophysical survey, a seismic source can be carried by a truck and positioned at a predetermined location in an area of exploration. The seismic source can be a single axis vibratory source and can impart compressing P-waves into the earth once coupled to the earth and operated.
A vibrator 10 according to the prior art is illustrated in FIGS. 1A-1C and is diagrammatically illustrated in FIG. 2. The vibrator 10 transmits force to the ground using a base plate 20 and a reaction mass 50. As is typical, the vibrator 10 is mounted on a carrier vehicle (not shown) that uses a mechanism and bars 12/14 to lower the vibrator 10 to the ground. With the vibrator 10 lowered, the weight of the vehicle holds the base plate 20 engaged with the ground so seismic source signals can be transmitted into the earth.
The reaction mass 50 positions directly above base plate 20 and stilts 52 (FIG. 2) extend from the base plate 20 and through the mass 50 to stabilize it. Internally, the reaction mass 50 has a cylinder 56 formed therein. A vertically extending piston 60 extends through this cylinder 56, and a head 62 on the piston 60 divides the cylinder 56 into upper and lower chambers. The piston 60 connects at its lower end to a hub in a lower cross piece 54L and extends upward through cylinder 56. The piston 60's upper end connects to a hub on an upper cross piece 54U, and the cross pieces 54U-L connect to the stilts 52.
To isolate the base plate 20 from the bars 14, the bars 14 have feet 16 with isolators 40 disposed between the feet 16 and the base plate 20. As shown, three isolators 40 are disposed under each foot 16. In addition, the feet 16 have tension members 42 interconnected between the edges of the feet 16 and the base plate 20. The tension members 42 are used to hold the base plate 20 when the vibrator 10 is raised and lowered to the ground. Finally, shock absorbers 44 are also mounted between the bottom of the feet 16 and the base plate 20 to isolate vibrations therebetween.
FIGS. 3A-3C show the base plate 20 for the prior art vibrator 10 in plan, side, and end-sectional views. The top of the plate 20 has stilt mounts 24 for the stilts (52; FIG. 2), and a reinforcement pad 21 surrounds these mounts 24. Retaining ledges 26 are provided for the isolators (40). The long edges near the corners have forked hangers 28 to which ends of the tension members (42) connect, and reinforcement pads 27 are provided around the outside edges of the plate 20 for connecting the shock absorbers (44) to the base plate 20.
Overall, the base plate 20 can have a height H1 of about 6.9-in., a width W1 of about 42-in., and a length L1 of about 96-in., and the plate 20 can weight approximately 4020-lbs. As shown in the end section of FIG. 3C, the plate 20 has four internal tubes or beams 30 that run longitudinally along the plate's length. The beams 30 are hollow tubes with rectangular cross-sections and have a height of about 6-in., a width of about 4-in., and a wall thickness of about ⅜-in. Interconnecting spacers 32 position between the beams 30 and between the long cap walls of the base plate 20.
During operation, a controller 80 as shown in FIG. 2 receives signals from a first sensor 85 coupled to the upper cross piece 54U and receives signals from a second sensor 87 coupled to the reaction mass 50. Based on feedback from these sensors 85/87 and a desired sweep signal for operating the vibrator 10, the controller 80 generates a drive signal to control a servo valve assembly 82. Driven by the drive signal, the servo valve assembly 82 alternatingly routes high pressure hydraulic fluid between a hydraulic fluid supply 84 and upper and lower cylinder piston chambers via ports in the mass 50. As hydraulic fluid alternatingly accumulates in the piston's chambers located immediately above and below the piston head 62, the reaction mass 50 reciprocally vibrates in a vertical direction on the piston 60. In turn, the force generated by the vibrating mass 50 transfers to the base plate 20 via the stilts 52 and the piston 60 so that the base plate 20 vibrates at a desired amplitude and frequency or sweep to generate a seismic source signal into the ground.
As the moving reaction mass 50 acts upon the base plate 20 to impart a seismic source signal into the earth, the signal travels through the earth, reflects at discontinuities and formations, and then travels toward the earth's surface. At the surface, an array of geophone receivers (not shown) coupled to the earth detects the reflected signal, and a recording device records the signals from the geophone receivers. The seismic recorder can use a correlation processor to correlate the computed ground force supplied by the seismic source to the seismic signals received by the geophone receivers. The seismic source has a hydraulic pump subsystem with hydraulic lines that carrying hydraulic fluid to the servo valve assembly 80, and a cooler may be present to cool the hydraulic subsystem.
When operating such a prior art vibrator 10, operators experience problems in accurately determining the ground force that the vibrator 10 is applying to the earth and in accurately correlating the vibrator's operation with the generated source signal. Ideally, operators would like to know the actual ground force applied by the base plate 20 to the ground when imparting the seismic energy. Unfortunately, the base plate 20 experiences a great deal of vibration and flexure that distorts readings that can be obtained from the base plate 20. Moreover, the isolators 40, shock absorbers 44, and other components required to isolate the base plate 20 from the supports 14 and feet 16 limit what free and unencumbered space is available on the base plate 20 to obtain acceleration readings.
For these reasons, a local sensor (e.g., accelerometer or geophone) is typically positioned on the upper cross piece 54U of the vibrator 50, which positions above the reaction mass 50 as best shown in FIG. 1C. Affixed at a location 55 on the upper cross piece 54U, the accelerometer (85; FIG. 2) couples to the base plate 20 via the stilts 52. In this location on the upper cross piece 54U, the accelerometer (85) can avoid the flexure, undesirable noise, distortion, and the like that occurs at the base plate 20, while still measuring acceleration for the base plate 20. For this reason, typical work in the prior art to improve performance of such a vibrator 10 has focused on optimizing the location of the local sensor (85) on the upper cross piece 54U to avoid issues with noise, flexure, and other problems.
In operation, the controller 80 shown in FIG. 2 measures the signal imparted into the earth using the local sensor 85 located on the upper cross piece 54U and the sensor 87 located on the reaction mass 50. The measured signals are transmitted to a correlation processor, which also receives the signals from geophones or other sensors making up the seismic spread. The correlation processor uses various algorithms to distinguish wave signal data from distortions and other spurious signals. A problem with this method is that the original source signal distortion may vary making correlation difficult. Thus, the cleaner the source signal imparted into the earth the easier the correlation at the recording end of the seismic acquisition process. Also, the more accurate the source signal is, the more energy the source vibrator 10 can impart to the earth.
Because the vibrator 10 works on the surface of the earth, which can vary dramatically from location to location due to the presence of sand, rock, vegetation, etc., the base plate 20 is often not evenly supported when deployed against the ground at a given location. In addition, the base plate 20 will flex and directly affect the control system during operation. As a result, the radiated energy produced can vary from location to location depending on where the vibrator 10 is deployed. Therefore, the vibrator's source signature is not the same (or nearly the same) from location to location and is not characteristically repeatable, which is desirable when performing seismic analysis.
When calculated ground force signals at the vibrator 10 are cross-correlated with far-field signals measured in the field, it has been recognized in the art that locating an accelerometer on a base plate can cause errors in the arrival time of the seismic waves. One theoretical approach proposed in the prior art for reducing the time shifts caused by the phase lag in the oscillations of the base plate 20 relative to the actuator force from the piston 60 suggests locating the accelerometer on the base plate at a radius that is approximately 68% of the plate's total radius. See A. Lebedev and I. Beresnev, “Radiation from flexural vibrations of the baseplate and their effect on the accuracy of traveltime measurements,” Geophysical Prospecting, 2005, 53, 543-555. Yet, it has also been recognized that it may be practically difficult to find the exact location of the base plate for the accelerometer to improve the time shifts so that a more practical solution would be to modify the resonance of the base plate 20 so that problematic modes of this resonance would lie above an upper frequency of a sweep signal used during seismic survey. Although this approach may be affective, the subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.